1. Field of the Invention
This invention relates to a method of reconfiguring an oil tanker. More particularly, this invention relates to a method whereby a standard single hull tanker can be converted into a so-called mid-deck tanker in order to limit the quantity of oil spilled in the event of a collision or grounding.
2. Description of the Prior Art
Increasing public awareness of the fragile nature of the Earth's environment coupled with seeming increase in the number of oil tanker accidents has resulted in substantial world wide attention to safe transport of oil in tankers, that is, in specialized ocean going ships carrying extremely large amounts of crude oil from production locations such as the Middle East to refinery locations such as in the United States.
In the past, oceanic shipment of oil has been made in single hull tankers, in which the midship cargo section (that is, located in the middle part of the hull) of the tanker is divided by longitudinal and transverse bulkheads into a series of port and starboard wing tanks and center tanks. The outer hull plating of the ship forms the outer shell of the wing tanks. Similarly, the bottom plating forms the lower shell of the central tanks. Thus, no ballast tanks, void spaces or the like are present between the hull plating and the tanks containing the oil. The obvious defect of such "single-hull" tankers is that any damage to the hull will typically cause the oil in the corresponding tanks to be released, possibly causing damage to coastlines, wildlife, and fisheries.
In this connection it is sometimes argued that oil carried in a tanker holed in an accident will not necessarily drain out of a punctured tank, because the oil, being lighter than water, will float. However, it will be appreciated that tankers are most commonly holed by running aground. This typically occurs near harbors, shorelines, or the like where there are usually substantial currents and tides. The action of tidal or current flow past an open puncture in an oil tank causes sufficient turbulence to pull the oil out of the tank, despite the fact that the oil is lighter than water.
Therefore, the public has recognized, as has the art, that improvements in tanker design are necessary to eliminate or limit as much as practical the risk of oil pollution caused by tanker accidents.
FIG. 1 shows an exemplary cross-sectional view through the midship cargo section of a typical single-hull tanker, looking forward. As there shown, the midship section is defined by port outer plating 20, starboard outer plating 22, a port longitudinal bulkhead 24, a starboard longitudinal bulkhead 26, hull bottom plating 28, and deck plating 30. Evidently, if the hull bottom plating 28, for example, is pierced by running the tanker aground, oil in the tank including that portion of the hull bottom plating 28 can leak out.
Accordingly, there have been built so-called "double-hull" tankers as depicted in schematic cross-sectional midship view in FIG. 3, again looking forward. There the outer hull plating includes port outer plating 32, starboard outer plating 34, hull bottom plating 36, and deck plating 38, all essentially in as the case of the single-hull tanker of FIG. 1. In this case, however, the port and starboard longitudinal bulkheads, 40 and 42 respectively, do not extend to the hull bottom plating 36 but terminate in a horizontal mid-deck 44 extending laterally between a outer port longitudinal bulkhead 46 and a outer starboard longitudinal bulkhead 48. Thus the midship section of the double-hull tanker of FIG. 3 includes a port wing cargo tank 50, a center cargo tank 52, and a starboard wing cargo tank 54; these cargo tanks are separated from the corresponding sections of the hull plating 32, 34 and 36 by ballast tanks 56 and 58. Ballast tanks 56 and 58 are normally filled with seawater when the oil tanks 50, 52 and 54 are empty, e.g., during a return voyage, and are emptied when the oil tanks are filled for an outbound voyage. In this way the vessel's draft and trim are maintained appropriately.
Segregation of the cargo tanks from the ballast tanks according to the double-hulled tanker design has many significant advantages. A severe impact can be absorbed without puncturing an inner cargo tank, due to the substantial distance between the outer hull plating and the cargo tanks. Moreover, by thus segregating the ballast and oil tanks, ballast water can be pumped into and subsequently removed from the ballast tanks without becoming contaminated with oil. This eliminates the problem of oil mixing with ballast water. By comparison, commonly some of the cargo tanks of a single-hull tanker are filled with ballast water during the return voyage, to maintain the trim of the vessel. When the ballast is subsequently pumped out to accept further oil for a subsequent outbound voyage, as has been common practice, substantial pollution may result, as the ballast water tends to become contaminated with residual oil in the tanks.
Despite its advantages (and certain presently contemplated international regulations), the double-hulled tanker design has not yet been accepted universally by the oil transport industry, principally because for a given deadweight a double-hulled tanker's cargo capacity is marginally less than that of a single-hull tanker, while the cost of the double-hulled tanker is marginally greater. In particular, substantial tonnage of single-hull tankers is now in service. The owners of these ships clearly do not wish to scrap these tankers and will not replace them with costly double-hulled tankers unless they are obliged to do so. To date no effective international organization or government has made double-hulled tankers mandatory.
The art is also aware of a so-called "mid-deck" tanker design shown in midships cross-sectional view in FIG. 2, again looking forward. The mid-deck tanker is comparable to the double-hulled tanker of FIG. 3 in that wing cargo tanks 60 and 62 on either side of central cargo tanks 64 are spaced from the outer hull plating 66 and 68 by ballast tanks 70 and 72 extending into the chines of the vessel. However, the central portion of bottom plating 61 of the mid-deck tanker still forms the outer shell of a lower central cargo tank 78. The "mid-deck" characterization of the tanker of FIG. 2 is due to the presence of a mid-deck 80 dividing the upper and lower central tanks 64 and 78. Perforated extensions 82 and 84 of the mid-deck 80 may also be provided.
The advantage of the mid-deck tanker design of FIG. 2 as compared to the full double-hulled design of FIG. 3 is that a mid-deck tanker of particular dead weight has increased cargo carrying capacity since the lower central tank 78 contains cargo rather than ballast. Moreover, the mid-deck tanker can be effectively converted to a double-hulled tanker simply by carrying ballast rather than oil in the central lower tank 78, should international shipping regulations someday so require. As compared to the single-hull tanker of FIG. 1, the mid-deck tanker of FIG. 2 provides substantially increased protection against accidental oil spills, since the wing ballast tanks 70 and 72 would absorb most impacts; that is, it is found that most accidents cause damage to the outer hull plating 66 or 68 or the plating of the chines, rather to the central bottom plating. Further, the presence of the mid-deck 80 dividing the central tank into upper and lower sections 64 and 78 limits the amount of oil spilled if the bottom plating is holed. Thus for a given amount of hull damage the mid-deck tanker of FIG. 2 will spill less oil than a single-hull tanker as in FIG. 1.
As indicated above, there is a substantial possibility that governmental or international regulation may shortly outlaw single-hull tankers as in FIG. 1. This would require the owners of those tankers to scrap them within some fixed period of time and replace them with the double-hulled tankers of FIG. 3 in order to remain in the oil shipping business. Interim regulations may allow the use of mid-deck tankers for oil transport. It would be advantageous to provide a method whereby such single-hull tankers could be economically reconfigured as safer vessels, thus extending their useful life at reasonable expense, while serving the end of reducing oil pollution caused by tanker collisions or accidental groundings.